Methods for assembling prepreg stacks having exact weight for producing SMC components

ABSTRACT

A method for manufacturing SMC components from an appropriately adapted amount of a fibrous reactive synthetic resin which is provided in the form of defined blanks of prepregs. The prepregs are placed in a defined position into a heated mold of a molding press, the heated resin/fiber mass is flow-molded in the closing mold to form the SMC component, said component is thermally cured and subsequently removed from the mold. Two different approaches are taken, one being intended for introducing the resin mass into the mold as multiple layers and the other for introducing the resin mass as a single layer.

This application claims the priority of German Patent Document No. 10145 308.6, filed 14 Sep. 2001 and PCT/EP02/09095 filed 14 Aug. 2002 thedisclosure of which is expressly incorporated by reference herein,respectively.

FIELD OF THE INVENTION

The invention relates to a method for producing SMC components fromfibrous, reactive prepregs.

BACKGROUND OF THE INVENTION

An article by R. Brüssel and U. Weber “SMC-Teile vollautomatischherstellen” [Fully automatic production of SMC components], published inthe journal Kunststoffe, year 79 (1989), pages 1149-1154—cited hereafteras [1] for short, describes a method for forming SMC components.

According to the literature reference [1], the production of SMCcomponents starts with a specific amount of a mixture of reactivethermosetting synthetic resin and fibers that is adapted in its weightto be appropriate for the finished component. To be precise, the adaptedamount of raw material is obtained by cutting out blanks of a specificsize and shape from a prepreg web supplied in roll form and by layingthe blanks together to form a prepreg stack. Such a prepreg stack isplaced exactly in position in an opened mold of a press. The mold isheated to a temperature at which the reactive synthetic resin chemicallyreacts and sets. By initial slow closing of the mold located in thepress, the raw material introduced is at first merely heated, wherebythe synthetic resin becomes soft and free-flowing. Subsequently, themold is closed with a controlled force and speed, the softened rawmaterial flowing away to the sides and thereby completely filling thecavity of the mold. After this filling of the impression, the mold iskept closed for a time with a defined force, so that the synthetic resincan fully react and cure. Only then can the mold be opened and thefinished SMC component be removed from it.

In the article [1] cited at the beginning, reference is made inter aliato a varying basis weight of the prepregs. In spite of all the effortsof the prepreg manufacturers, according to [1] even today it is stillnot possible for the prepreg webs to be manufactured with sufficientaccuracy in the basis weight. Therefore, in preparation for eachmanufacturing step of an SMC component, it must be ensured that the massof prepregs introduced into the mold is always the same, at least withina certain tolerance range. The higher the quality requirements imposedon the finished product, the less the resin mass introduced may varyabout a desired value. In [1] it is mentioned that the problem of thevarying basis weight of the prepreg web, and the consequent problem ofexact feeding of the raw mass, could be overcome if the qualityrequirements imposed on the finished SMC molding could be reduced. If,however, the SMC components to be manufactured are thin-walled shellcomponents with high quality requirements, the mass of the raw materialto be introduced should wherever possible be fed in with a low range ofupward and downward variation in comparison with a setpoint selection.If the amount of raw material introduced is too small, this causes theformation of surface roughnesses and also thin and weak points in thecomponent, which in an extreme case could become perforated. If, on theother hand, too much raw material is fed into the mold, the wallthickness becomes too great, at least locally, which under somecircumstances leads to warping of the component; in any event,components with excessive material are not dimensionally stable enough.Furthermore, in the case of overfeeding, material swells out along theparting line of the mold, which leads not only to excessive flash andcorresponding extra work to remove the flash, but also to increasedsoiling of the mold and consequently an increase in the secondary workof “mold cleaning”; that is overfeeding leads overall to a reduction inproductivity.

In the case of the automated method for manufacturing SMC componentsdescribed in the literature reference [1], the blanks arranged in layersto form a prepreg stack as a raw mass are all rectangularly shaped andall have the same width in one direction, lying transversely to theprepreg web, that is the width of the prepreg web itself trimmed at theedges. The blanks are produced by cutting across the prepreg web, usinga pneumatically driven high-speed cutter that is moved transversely overthe prepreg web, which is supported at the location of the cut by anarrow profile. The high-speed cutter presumably leaves the prepreg webto be cut on the underside and enters a longitudinal slot in thesupporting profile. To compensate for a changed basis weight of theprepreg web, the rectangular dimension of the blanks in the longitudinaldirection of the prepreg web is used. For monitoring the target weightto be maintained of the prepreg stack, it is not the cut-off blanks thatare weighed but the finished SMC component. Depending on the deviationof the finished weight of the SMC component from a desired weight, theblanks are cut longer, shorter or the same for the next SMC component tobe produced. A fundamental disadvantage of the control strategy knownfrom [1] for maintaining the desired weight of the raw mass to beintroduced is that a control intervention for correcting the actualcontrolled variable—raw mass for the component n—is made dependent onthe desired/actual deviation of a variable other than the measuredvariable—that is the finished component mass of the component n+1. Themeasured variable “finished component mass of the component n+1” doesnot by any means have to be representative of the actual controlledvariable “raw mass for the component n”. The method described in [1]attempts to record or predict possible differences between thecontrolled variable and the measured variable by continuously recordingthe thickness of the prepreg web. Because of the differences between themeasured variable on the one hand and the controlled variable on theother hand, a high proportion of the SMC components manufacturedaccording to deviate from the desired weight aimed for; the controlstrategy known from [1] only works on the basis of such a deviation.Apart from this, in the method according to [1] the blanks have to berectangular, with a width corresponding to the width of the prepreg web.However, this prerequisite can only be allowed optimally in terms of themethod for a restricted spectrum of components.

In conventional methods for manufacturing series of SMC components,often performed manually, the mass of prepregs introduced into the moldis individually weighed, which likewise takes place manually andconstitutes a great obstacle to automation of the process. This usuallyinvolves cutting out rectangular blanks with a sharp knife from avirtually endless prepreg web on a steel base and weighing them. If thedesired weight of a blank to be introduced into a prepreg stack is toogreat, an edge strip is cut off on one longitudinal side of a blank or atriangular piece is cut off at a corner, whereby the desired weight isachieved approximately but not exactly. In particular, however, thedesired shape of the blank is greatly changed by such a correction,which has disadvantageous effects on the molding process and thecomponent quality. If, on the other hand, the desired weight of a blankis too low, the next-following blank is cut somewhat larger than thedesired shape or a small trimmed-off part of an earlier correctiveoperation is added. These types of correction have disadvantageouseffects on the subsequent molding operation and the component quality.Moreover, this manual weighing of the amount of raw material means thatthe desired weight is only approximated with a very great range ofvariation, which is scarcely any less than the weight variation of theprepregs themselves. For this reason, in the production of SMCcomponents with manual weighing of the raw material there are arelatively high number of reject components and relatively considerablequality variations.

EP 461 365 B1—cited hereafter as [2] for short—discloses a method formanufacturing plastic moldings from thermoplastic material in which anamount of heated and softened thermoplastic material appropriatelyadapted in weight is placed into an opened mold of a press, the moldingcompound is forced to flow into the cavity of the mold by closing themold and subsequently the workpiece still located in the mold is cooledand finally removed from it. The special feature of the method describedin [2] is the preparation of the heated thermoplastic material in a flatpreform already appropriately adapted approximately to the shape of thecavity of the mold, with the distribution of the compression moldingcompound within the preform also already having been approximatelyadapted to the requirements of the cavity of the mold. For this purpose,a thin, wide strand of extrudate of hot molding compound is deposited onthe heated and reversibly drivable conveyor belt of a belt weigher andat the same time weighed. The strand of extrudate is deposited on theconveyor belt in a meandering manner and with a variable massdistribution on account of a slow oscillating motion of the conveyorbelt in the conveying direction or counter to it and on account of aspecific belt speed, which may deviate from the extrusion speed. Also inthe case of this method for the compression molding of thermoplasticmaterial, the molding compound to be introduced into the cavity of themold is to correspond exactly to a desired weight, in order on the onehand to ensure complete filling of the cavity and on the other hand topermit complete closing of the mold without excessive formation offlash. In the case of the method shown in [2], this is achieved by thestrand of extrudate deposited on the belt weigher being continuouslyweighed. When, toward the end of the formation of a preform, part of thestrand of extrudate is still hanging from the extruder die and not theentire molding compound intended for the preform is exerting its weighton the weigher, the extruded strand is cut off just before the desiredweight is reached, i.e. when a certain weight threshold is reached. Ifthe weight of the preform that is then completely on the belt weigherlies within a predetermined tolerance range, it is passed on to adownstream conveying devices, which deposit the preform into the openedmold. If, on the other hand, the formed preform is too heavy or toolight, it is rejected and its molding compound is recycled. The weightthreshold for triggering the severing of the strand for thenext-following preform is also correspondingly corrected, i.e. in thecase of an excessively heavy preform the weight threshold is changed inthe direction of a lower threshold weight, and vice versa. However, thistype of control of the weight of the molding compound to be introducedinto a mold cannot be transferred to the processing of fiber-reinforcedthermosetting materials, i.e. it cannot be transferred to the portioningof prepreg blanks.

Japanese laid-open patent application JP 10 044 153 A, cited hereafteras [3] for short, discloses a method for preparing prepreg blanks forthe manufacture of SMC components. This involves processing individualpieces of material web of a length adequate for forming a number ofrectangular blanks and of a width which coincides with the width of theblanks. At the beginning of the processing of a material web, a firstrectangular blank of a known length is cut off, this first blank isweighed and this is used to determine the weight per unit length of thematerial web. On the basis of this weight per unit length, assumed to besufficiently constant within the piece of material web to be processedat the time, a length of blank is mathematically fixed for the blankssubsequently to be cut off with the same surface area from the materialweb, with which length the blanks have a weight which coincidesapproximately from one to the another and also corresponds withsufficient accuracy to the weight required for the workpiece to beproduced. Subsequently, the other pieces of material web aretransversely cut in a way corresponding to this fixed specification intopieces of the same length and the blanks prepared in this way areprocessed one after the other in an SMC molding press for workpieces. Adisadvantage of this procedure is that it is only possible in this wayto process relatively short pieces of material web, for which the basisweight or the weight per unit length can be regarded as constant withsufficient accuracy over the entire length of the piece of web.

SUMMARY OF THE INVENTION

An object of the invention is to provide a method of producing SMCcomponents wherein, in spite of a varying basis weight of the prepregweb, the desired weight prescribed for the prepregs can be maintainedwith high accuracy for every production cycle of an SMC component,without significantly changing the shape of the individual blanks.

This and other objects and advantages are achieved according to theinvention in the following embodiments.

In an embodiment, in every working cycle a reference blank with constantshape and size is cut to size and separately weighed each time. Theweight and size of the reference blank and the aimed-for total weight ofthe prepreg stack are used to determine mathematically the surface-areasize of corrective parts which are cut to obtain the desired weight ofthe resin mass to be introduced. This is based on the assumption thatthe basis weight of the prepreg web changes only by a negligible amountin the direct vicinity of the location at which the reference blank hasbeen cut out from the prepreg web. In such an embodiment, which is basedon a resin mass comprising a stack of multiple layers, the furtherprepreg layers are regarded as corrective parts and the size to bemaintained by all of them is determined. These other blanks are then cutout from a region of the respective prepreg stack lying directlyadjacent to the reference blank in the prepreg web and formed into astack.

In another embodiment wherein the prepreg is to be introduced as asingle layer, a reference part is cut out with excess size and weighed,and the excess in terms of weight is then cut off in a correspondingsurface area.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic overall view of an installation for the methodin a plan view,

FIG. 2 shows the cutting table with the outline on a prepreg web for thecutting to size of the parts of a seven-part prepreg stack of a firstexemplary embodiment,

FIG. 3 shows an auxiliary device set up on a weigher for weighing thereference blank and for preparing a prepreg stack obtained according toFIG. 2,

FIG. 4 shows the cutting tool arranged on the hand joint of anindustrial robot, with a circular saw blade, which performhigh-frequency rotary oscillating movements, for cutting up the prepregweb supported by a glass plate,

FIG. 5 shows an enlarged detail of the cutting intervention of thecircular saw blade into the supported prepreg web and

FIG. 6 shows the cutting to size of a single-layer useful blank of apredetermined weight from a reference blank of varying basis weight as asecond exemplary embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMODIMENT

The method on which the invention is based for manufacturing series ofSMC components is to be briefly explained on the basis of the diagram ofthe method according to FIGS. 1 and 2. The SMC components aremanufactured from a fibrous, reactive resin mass, which is provided inthe form of a virtually endless prepreg web 22 wound up into a supplyroll 1 as an intermediate. The fibers contained in the prepreg web aregenerally glass fibers; in the case of heavy-duty SMC components, carbonfibers or Kevlar fibers may also be integrated. The fibers are cut andhave a length of approximately 1 to 5 cm. To maintain the reactivity ofthe synthetic resin in the prepreg web 22, the latter is covered on bothsides with a protective film 26, which is pulled off and rolled up toform a separate roll 2 only shortly before the processing of theprepreg. As can be seen more clearly in FIG. 2, the protective film isdeflected counter to the processing direction of the prepreg to the roll2 via a reversing rod 12, located in the vicinity of the cutting table3. The side edges of the prepreg web are unsuitable for furtherprocessing and must be cut off. The lateral waste strips 28 are likewisedeflected via reversing rods 13 into waste containers 14.

Alternatively, when the cutting tool described further below is used, itis also possible for the prepreg webs provided with an adhesivelyattached protective film to be cut. For example, the edge strips 28(FIG. 2) can be cut off before the protective film 26 is pulled off. Insome embodiments, it may be desired to cut the blanks to size with theprotective film, to allow them to be stacked up before furtherprocessing and only processed further at a later point in time. Theprotective film adhering to each of the blanks readily prevents theblanks that are stacked or deposited in an imbricated formation fromsticking together. Conventionally separate sheets of film were requiredas an intermediate layer, causing extra costs.

The following description is based on an embodiment, in which theprotective film is pulled off and wound up as one before the cutting ofthe prepreg web. This embodiment has the advantage that it can bechecked more easily whether the protective film has detached itselfcompletely from the prepreg web. Remains of film adhering to the blanksare detrimental to the further processing process and to the strength ofthe molding to be manufactured.

In one embodiment, the prepreg web 22 is cut up on the cutting table 3provided with a very hard support. Blanks of a defined shape and sizeare cut out from the prepreg and stacked up to form a multi-layerprepreg stack web of a specific number of layers and arrangement oflayers. The trimmed-off parts produced thereby, which cannot be used anyfurther, are removed into a corresponding waste container 4. The cuttingto size may in principle be performed manually with a sharp knife and asteel rule. In the case of the exemplary embodiment represented in thefigures, however, a mechanized and automated cutting to size by means ofa cutting robot 5 is provided. This embodiment is discussed in moredetail below.

On a separate weighing and stack-forming device 6—see also FIG. 3—theblanks cut by the robot 5 on the table 3 are stacked up to form aprepreg stack 31, the blanks being handled and moved by a handling robot7, which for its part is equipped with a prepreg gripper 27 designedspecifically for this task and this substrate. Once the prepreg stack 31has been formed in an appropriate shape for a new workpiece, it isplaced by the handling robot in a defined position into a heated mold 35of the molding press 8.

The mold is closed by the press until the molding surface of the cavityis in contact with the placed-in prepreg stack and is clamped in theclosing sense by a defined, initially small force. The contact with thehot mold causes the resin mass to be heated and softened as a result. Onaccount of the closing force of the mold 35, the resin mass begins toflow and, as a result, fills the cavity of the increasingly closingmold. The mold is subsequently held in the closed state with anincreased force for a certain time, the resin mass thermally curing.Once this curing time has elapsed, the press 8 opens the mold, with thefinished SMC component remaining in the lower, fixed mold half. The SMCcomponent can be removed from the press and deposited in a coolingstation 11 by a removal robot 9, which is provided with a removal tool29. While the cutting and handling robots 5 and 7 prepare a new prepregstack, the opened mold is cleaned by two cleaning robots 10, so that itis ready for receiving a new prepreg stack.

The described method for producing the SMC components requires that theresin mass introduced into the mold coincides with a specific desiredweight. The cavity of the mold should be completely filled by the resinmass introduced, but on the other hand there should not be too muchresin mass in the cavity, because otherwise the mold cannot closecompletely and the formed SMC component is not molded to the truedimensions. Furthermore, an unnecessarily large amount of resin massswells out at the parting line of the mold, which makes the cleaningoperation more difficult. Unfortunately, the prepreg web 22 hasinadmissibly high variations in the basis weight, which cannot beavoided in the manufacture of this intermediate. Consequently, toachieve the prescribed desired weight, it is not possible to use blanksof a constant surface area from one working cycle to the next. Rather,special efforts have to be undertaken to achieve the target weight ofthe resin mass to be introduced in each working cycle.

To be able to maintain with great accuracy the desired weight prescribedfor the prepreg stack to be introduced for each production cycle of anSMC component, without significantly changing the shape of theindividual blanks, the present invention provides a method for cuttingthe blanks of the prepregs to size. This is based on the largely correctassumption that the basis weight of the prepreg web changes only by anegligible amount within a surface-area region required for the amountof resin mass needed for an SMC component.

On the basis of this finding, the blanks 24, 25 to be used for theprepreg stack 31 of a specific SMC component are not only cut out fromone and the same prepreg web 22, but also cut out from it directlyadjacent to one another. Furthermore, a special blank, a reference blank24, with a shape that is always constant and the same surface-areacontent Fr for all the prepreg stacks following one another is cut tosize and separately weighed each time after the cutting to size, and itsactual weight G_(r,actual) is determined. As a result, the local, actualbasis weight of the prepreg web is to a certain extent known. Apart fromcomprising the reference blank 24 of a constant surface area, therequired prepreg stack comprises further blanks 25 formed with variablesurface areas. These can be specifically dimensioned in their surfacearea in such a way that the total weight of the prepreg stack can betrimmed exactly and in a single operation to the desired weight that isto be maintained.

The respective weight G_(r,actual) and the surface-area content F_(r) ofthe reference blank 24 as well as the predetermined total weight G_(g)of all the blanks 24, 25, that is always the same for all the prepregstacks 31 following one another, are used to determine the surface-areacontent F_(u) to be maintained by all the other blanks 25 whichindividually correspond to the weighed reference blank 24, in accordancewith the relationshipF _(u) =F _(r)·(G _(g) /G _(r,actual)−1)or in accordance with a relationship which is derived therefrom andidentical in principle. With the knowledge of the magnitude of thesurface-area content F_(u) of the other required blanks 25, the lattercan be cut out from the prepreg web in an exactly specific manner withrespect to their weight together. These blanks are cut out from a pieceof the surface area lying directly adjacent to the reference blank 24 inthe prepreg web 22, appropriately adapted in shape and size and with thesurface-area content F_(u). The prepreg stack 31 assembled with theweighed reference blank 24, of a constant surface area, and with theother blanks 25, dimensioned individually in surface-area content, has atotal weight G_(g) that coincides with the desired weight to within afew tenths of a percent. In a laboratory trial carried out by theapplicant for the invention, it was possible to maintain thepredetermined desired weight of the prepreg stack to within a range ofvariation of ±0.3%. The prepreg stack of constant weight formed in thisway can consequently be readily placed in a defined position into themold 35 ready to receive it of the molding press 8 for furtherprocessing.

The continuous weighings of reference blanks of the same surface area,carried out when the method according to the invention is performed in aseries mode, incidentally also provide a reliable volume of data withrespect to the variation in the basis weight of the prepreg web in thelongitudinal direction of the web. The very dense set of data generatedcan be evaluated in various respects. For example, the basis weight ofthe prepreg web in the longitudinal direction of the web can be printedout as a longitudinal profile, i.e. as a line trace; the mean basisweight, the standard deviation and the maximum deviation from the meanvalue can be determined. These data allow reliable quality control orquality monitoring of the prepreg webs delivered.

As already mentioned, for each workpiece the total weight G_(g) of theprepregs is entered as a desired value into a mathematical operationwhich is then used for cutting the prepreg parts to size exactly andindividually for each workpiece. The invention readily allows thisdesired value to be slightly changed if appropriate or to be adapted tonew findings or circumstances. The desired weight can be corrected inthe course of series production from a value X to a value of, forexample, X+0.5% or, for example, to a value X−1.3%. With the input ofthe new desired weight, the actual total weights of the prepreg stacksmanufactured after the change are then also correspondingly higher orlower, to be precise likewise with the accuracy mentioned of ±0.3%. Theinvention therefore allows sensitive and exact selection of the totalweight of the prepregs that are to be placed into the mold 35.

Once the prepreg stacks have been assembled on a weigher, it is readilypossible also to determine exactly the actual total weight of thefinished prepreg stack by weighing, and to fix it for each workpiece.This not only allows monitoring of the method according to the inventionof weighing the prepreg webs. The actual weights of the finished prepregstacks that are fixed individually for each workpiece also provideimportant data for quality monitoring of the production of moldings.

Based on the admissible assumption made that the gradient of the basisweight within the prepreg web 22 is small, it follows that the basisweight is constant with sufficient accuracy within the amount of surfacearea needed for a prepreg stack. This in turn leads to therecommendation to cut out the reference blank 24 and the other blanks 25of each prepreg stack next to one another from the prepreg web,transversely to the longitudinal direction of the prepreg web, in such away that the space required in the longitudinal direction of the prepregweb is as small as possible.

In the case of a prepreg stack in which the other blanks 25 are formedsuch that they are rectangular and also congruent, as provided forexample in the case of the exemplary embodiment represented in FIGS. 2and 3, one dimension l of the side of the rectangle of the other blanks25 is left unchanged for all the prepreg stacks 31 following oneanother. Only the dimension b transverse thereto of the side of therectangle of the other blanks 25 is dimensioned individually for theprepreg stack in question according to the technical teaching of thepresent invention. In the case of the exemplary embodiment representedin FIG. 2, with six other blanks 25, the unchanged longitudinaldimension l of the side of the rectangle of the other blanks is alignedparallel to the longitudinal direction of the prepreg web 22. Theindividually dimensioned width dimension b of the side of the rectangleof the other blanks is aligned transversely to the longitudinaldirection of the prepreg web 22. In the simplified case described here,the width b of the other blanks 25 can be individually fixed inaccordance with the relationship b=F_(r)·(G_(g)/G_(r,actual)−1)/l·n,where n denotes the number of other blanks 25. Once the actual weight ofthe reference blank 24 has been determined on the weigher 15, this valueis automatically entered in digitized form into the control system orthe movement program of the cutting robot 5, which then cuts out theother blanks 25 with the individual width b from the prepreg web 22 onthe basis of this input.

The space requirement evident from FIG. 2 for the seven blanks 24 and 25shown there cannot be evenly distributed over the width of the prepregweb 22. In the region of the reference blank 24, only material of thedimension A is used up in the longitudinal direction of the web, whereason the opposite side of the web material of the significantly greaterdimension 2·l is used up. To compensate for this, it is expedient tochange over the sides on which the reference blank 24 and the otherblanks 25 are arranged for the next-following cutting-to-size operation,so that the prepreg is used up evenly on the right and left. However,this has to be correspondingly taken into consideration in theprogramming of the sequence of movements of the cutting robot 5.

The usable width of the prepreg web 22 is at least slightly greater thanthe width B of the reference blank 24 plus three times the greatestwidth b of the other blanks 25. As a result, an edge strip 30 isgenerally obtained at one edge of the prepreg web as cutting loss and isdischarged into the waste container 4. Only in the extreme case of anextremely low basis weight of the prepreg web may this edge strip lossbe negligible.

For the sake of completeness, another possibility should also be pointedout, that of trimming the weight of the prepreg stack to the desiredweight value in a single operation by suitable surface-area dimensioningof the other blanks. The possibilities mentioned below largely depend onthe type and shape of the SMC component to be manufactured and therelated question as to the extent to which the form of the prepreg stackmay vary from workpiece to workpiece. To be precise, it is conceivableto cut out from the prepreg web a certain number, for example four, ofthe other blanks 25 for each prepreg stack likewise with the samesurface area, i.e. with a constant length and with an always constantwidth, and merely to dimension the remaining number, in the example two,of the other blanks 25 individually in width. These two individuallydimensioned blanks 25 would of course vary much more in their widthdimension b than if the weight compensation were evenly distributed oversix blanks 25. In the extreme case, it would even be possible to useonly one of the other blanks for such a weight compensation, which inthe case of a two-layer prepreg stack would in any case be unavoidable.

In the case of the exemplary embodiment represented in FIGS. 2 and 3,the prepreg stack to be formed altogether comprises seven blanks, thatis a particularly large reference blank 24 and six much smaller otherblanks 25, which are stacked up in two small stacks lying next to eachother on the reference blank 24 lying at the bottom. Irrespective of thearrangement of the reference blank in the prepreg stack to be formed,however, a blank that is as large as possible should be selected to formthe reference, so that the weight determined is also reliablyrepresentative of the basis weight of the prepreg web in the region ofthe web end that is being worked on at the time. The reference blankshould expediently have a size of approximately 20 to 60% of the totalsurface area of all the blanks of the prepreg stack 31. If it is toosmall, the weight and the surface area do not represent the local basisweight with sufficient accuracy. If, on the other hand, the referenceblank is too large, it may be that the weight compensation is notsuccessful in every case with the relatively small other blank or blankswithout trimming them excessively in the extreme case of a very highbasis weight. In such a case, it is then better to take away at leastpart of the excess prepreg weight from the reference blank itself. Thisis to be discussed once again further below in connection with a furtherexemplary embodiment according to FIG. 6.

In the procedure of the method it is favorable if the blank lying at thebottom in the prepreg stack 31 is adequate in size to allow it to beselected as the reference blank 24. After weighing the reference blank,no further handling operations are then necessary with it, i.e. theother blanks 25 can be stacked on the reference blank still lying on theweighing plate 16 of the weigher 15 to form the prepreg stack 31. Withblanks that are substantially congruent, the lowermost blank istherefore chosen as the reference blank and is cut to always the samesize of surface area. In the case of stacks with, for example, fiveidentical prepregs, this would be approximately 20% of the total weight.In the case of six or more prepregs in a stack, the two lowermostprepregs, for example, can both be chosen to be the reference blank, cutto always the same size of surface area and weighed together.

In the case of the exemplary embodiment represented in FIG. 3, thereference blank 24 is not only weighed when it is placed onto theweigher 15, but at the same time also pre-formed in a specific way, asexpedient later for placing the finished prepreg stack 31 into the mold.For this purpose, fastened on the weighing plate 16 of the weigher is astacking device 17, which permits staged pre-forming of the referenceblank by the handling robot and the prepreg gripper. The other blanks 25are stacked up on the lower or upper portion of the reference blankdeposited in stages.

In principle, the invention can also be put into practice in a manuallyoperated procedure for the method, in which for example the cutting outof the blanks is also performed by means of a hand-held knife and steelrule on a steel base and in which the blanks are manually handled by theworker. This manner of working is also occasionally still encounteredtoday in the series production of SMC components. The reference blank 24could be cut relatively precisely by using a template. Templates ofdifferent shapes could also be used for the other blanks 25, it beingautomatically output after the weighing of the reference blank whichtemplate from a finely graduated set is to be used. Cutting to size byusing punching tools, as are used for the cutting to size of leather ina flat press, is also conceivable, if appropriate with the assistance ofvibrators. For cutting the other blanks to size with flexibility intheir surface area, a set of finely graduated punching tools would thenhave to be kept available. Depending on the computed result, anindividually specified punching tool from the set would have to beissued and placed onto the prepreg web for punching out a blank.

However, in spite of all the care taken, manual cutting to size of theprepregs entails the risk of a greater surface-area tolerance andconsequently weight tolerance. Quite apart from this, this strenuouswork in the direct proximity of noxious fumes from the prepreg webs isonly admissible for any time with a protective mask and is thereforeeven more strenuous or laborious. To avoid such manually causedinaccuracies and the strenuous work, it is recommended to cut out boththe reference blank 24 and the mathematically determined cut-to-sizeareas of the blanks 25 by means of a robot-guided cutting tool.

Under some circumstances, a sharp cutter with an exchangeable bladecould be used as such a robot-guided cutting tool—in a way similar to inthe case of manual cutting to size—said cutter being moved through theprepreg with a drawing cut, i.e. at a shallow angle, although it wouldbe necessary to monitor that no fiber strands attach themselves to thecutting edge and disturb a clean cut. Because of this problem, ahigh-frequency rotary oscillating circular saw blade 21, which performssmall rotational displacements around a stationary central position, isrecommended in the present case as the cutting tool for the automatedcutting to size of the blanks by means of cutting robot 5. During thecutting to size, the prepreg web 22 is supported by a smooth, continuousbase that is free from joints, in the form of a thick glass plate 23,which is harder than the cutting teeth of the circular saw blade.

At the hand joint 18 of the cutting robot 5, a drive motor 20 for thecircular saw blade 21 is secured by means of a holding angle in such aposition that the hand joint axis 19 crosses the axis of the rotaryoscillating movement of the circular saw blade. The drive motor sets thecircular saw blade in rotary oscillations with approximately 20,000rotational displacements per minute via an integrated displacement gearmechanism. The cutting tool is guided along the desired cutting line insuch a way that the circumference of the circular saw blade touches theglass plate 23 with a small force during the cutting. The quite smallrotary oscillating displacements h performed by the circular saw bladeare indeed greater than the tooth pitch t, but smaller or slightlygreater than the thickness s of the prepreg web 22. The rotaryoscillating circular saw blade acts in a way similar to a compass saw,but with two fundamental differences. On the one hand, the sawing toolhas a displacement along a circular path which tangentially enters thematerial being cut and does not leave the material being cut on theunderside; the cutting displacements in the form of circular arcs areoriented at a very shallow angle with respect to the plane of theprepreg. On the other hand, even in the loosely lying state, thematerial being cut cannot follow the high-frequency oscillating movementbecause of inertia, so that the prepreg resting loosely on the glassplate can be cut through without any trouble. The advantage of thiscutting tool is not only trouble-free and low-wear working when cuttingprepregs to size, but also the possibility of being able to carry outcuts along tight curves with precision.

Also to be briefly discussed below in connection with the exemplaryembodiment represented in FIG. 6 is a variant of the method for theseries production of SMC components in which an in principlesingle-layer useful blank 33 of a prepreg 22′ is used and it is placedin a defined position into the heated mold of a molding press. Thefurther sequence of the method for the manufacture of SMC components isthe same in principle here as already described further above. Even ifin the actual case a smaller blank or else a number of them should beplaced onto the useful blank 33 locally and in a defined position, thiscase is also intended to be implied in the description andrecommendation which follows, although this additional, smaller blank isnot specifically mentioned below.

Even when useful blanks that in principle comprise a single layer andare of constant weight G_(n) are used, at first a reference blank 32 iscut to size with a shape and surface content Fr that is always the samefor all the SMC components following one another and separately weighedeach time after cutting to size, the actual weight G_(r,actual) in eachrespective case being determined. The shape and surface-area contentF_(r) of this reference blank are chosen such that the latter protrudesbeyond the useful blank on all sides in every case. Even assuming anextremely low basis weight of the prepreg web 22′, the reference blankis large enough to allow the useful blank 33 to be cut out from it withthe prescribed weight G_(n) of the useful blank 33. In any event, a moreor less large waste piece 34 is obtained when cutting back the referenceblank, i.e. when cutting to size the useful blank 33 from it.

In the case of the exemplary embodiment represented in FIG. 6, arectangular shape with the side lengths A′ and B′ is chosen for thereference blank 32, the width dimension B′ corresponding to the usablewidth of the prepreg web 22′. The reference blank can then be cut tosize by a straight cut taken transversely to the longitudinal directionof the prepreg web at the distance A′ from the previous end edge. Itjust has to be ensured that the surface-area content F_(r) of all thereference blanks are the same as one another with an error deviation ofvery few tenths of a percent. The cutting table 3′ used for cutting thereference blank 32 is at the same time formed as a weigher, i.e. theglass plate 23 forming the table top is at the same time the weighingplate of a weigher. For weighing the cut-free reference blank, the endof the prepreg web 22′ must be temporarily lifted off the cutting table3′ by the handling robot 7 or by another, more simple auxiliary device.Alternatively, it is also possible to restrict the weighing plate onlyto a partial region of the cutting table and, for weighing only thereference blank, to lower the weigher to such an extent that the end ofthe prepreg web is no longer touching the weighing plate, because it isheld by the surrounding table top.

The respective weight G_(r,actual) and the surface-area content F_(r) ofthe reference blank 32 and also the predetermined weight G_(n) of theuseful blank 33 are used for determining the surface-area content F_(a)of the excess in terms of surface area of the reference blank 32 incomparison with the surface-area content F_(n) of the useful blank 33,i.e. the size in terms of surface area of the waste piece 34. For this,the relationship F_(a)=F_(r)·(1−G_(n)/G_(r,actual)), or a relationshipwhich is derived therefrom and identical in principle, is used.

Once there is knowledge of the surface-area content F_(a) of the wastepiece 34, this item of data can be automatically entered in a suitable,for example digitized, form for each workpiece into the control systemof the cutting robot 5. Stored in the robot control system is a finelygraduated set of movement lines for guiding a curved cut, eachindividual movement line being assigned a specific surface area F_(a).Three of these cutting lines are indicated in FIG. 6. In accordance withthe output of a specific F_(a) value, the associated movement program isactivated in the control system of the cutting robot and the cuttingrobot is moved in accordance with it. The hatched region lying outsidethe actual cutting line represents the waste piece 34 to be removed. Ifthe reference blank is very heavy, a waste piece 34 with a largesurface-area content F_(a) is cut off, in the case of a light referenceblank the converse case applies. In any case, the waste pieces to be cutoff are similar in their shape and in any event a piece of surface areacoinciding in shape and size with the desired useful blank 33 with theweight G_(n) remains and can be placed into the mold.

In an embodiment of the method according to FIG. 6 that, if it has beencut to size by using an industrial robot, the reference blank is nolonger moved after the cutting to size, i.e. the reference blank shouldnot, for weighing purposes, be taken off the base on which it was cut orbe moved, because otherwise the reference to the system of the cuttingrobot is lost. This is also the reason why the cutting table is formedat the same time as a weigher or as a weighing plate.

It may happen under some circumstances that, because of a localthickening on a workpiece, a smaller blank also has to be placed locallyonto the useful blank. This additional blank would be ignored in themethod previously described in connection with FIG. 6 for the weightcorrection of the resin mass to be introduced by cutting back thereference part 32 specifically in terms of its surface area. If thisadditional part were always cut to size with the same surface area, acertain error would be included in the predetermination of the totalweight of an amount corresponding to its proportionate size. In orderhowever to allow such a smaller blank also to be included in thepredetermination of the total weight that is to be maintained,nevertheless only the weight of the bottom useful blank would have to beentered in the above relationship for determining the surface area F_(a)of the waste piece, instead of the total weight of the stack to beplaced into the mold. The size of the not-included further blank wouldhave to be cut out from the waste piece, it being necessary for thisblank to be cut to size with respect to its surface content in inverseproportion to the determined basis weight of the reference blank.

For the sake of completeness, a modification of the cutting methodaccording to FIG. 6 should also be mentioned, a useful blank 33 of thesame shape, trapezoidal in rough approximation, being assumed here. Thismodification of the process manages with a much smaller amount oftrimmed-off waste. To be precise, on the one hand the useful blanks thatare trapezoidal in rough approximation would have to be placed into theprepreg web in such a way that the two straight and mutually parallelside edges of the useful blanks come to lie parallel to the side edgesof the prepreg web; the feed of the prepreg web would consequently haveto be imagined from the left or right side in FIG. 6. Moreover, thewidth of the prepreg web would have to be chosen such that it coincideswith the corresponding dimension of the largest required usefulblank—which is represented in FIG. 6 by a dash-dotted line; it wouldalso be conceivable to arrange two sequences of reference parts parallelnext to one another on the prepreg web. On the other hand, the usefulblanks following one another in the longitudinal direction of the webare cut out from the prepreg web alternately from one side then theother, so that the large dimension, parallel to the longitudinaldirection of the web, of one useful blank always coincides with thesmall dimension of the next-following useful blank.

In the case of such a variant of the method, the reference blank that isto be used is a blank already cut to size in a way corresponding to thedesired free form, which is dimensioned to be large enough to be justright with respect to its surface-area content in the case of the lowestbasis weight of the prepreg web, i.e. the basis weight lying at thelower end of the range of variation, and has the required desiredweight. By temporarily lifting the end of the prepreg web off thecutting table, which is at the same time the weighing plate, it ispossible to determine the actual weight of this reference part, which isgenerally too high in comparison with the desired weight to bemaintained. In a way similar to that already described in connectionwith FIG. 6, the excess weight of the reference part in question isconverted into a new blank contour, three of which are indicated in FIG.6 by differently drawn line traces. The reference part is specificallyreduced in weight by a more or less wide edge trim similar to theoutline shape of the reference part and, as a result, the desired weightof the useful blank 33 is exactly brought about. Apart from theform-dependent trim, only a relatively narrow, weight-dependent edgestrip is then obtained as trimmed-off waste.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

1. A method for manufacturing a plurality of SMC components comprising:cutting a reference blank having a reference blank shape, a referenceblank size, and a reference blank surface-area content; obtaining areference blank weight; determining either a proper surface-area contentor an excess surface-area content of a single prepreg web used toproduce a plurality of non-reference blanks using said reference blanksurface area content, said reference blank weight, and a predeterminednon-reference blank weight; removing a waste piece having said excesssurface area from said single prepreg web; cutting said reference blankand said plurality of non-reference blanks from said single prepreg web,wherein the blanks are directly adjacent to one another in the prepregweb prior to cutting; stacking said reference blank and said pluralityof non-reference blanks to form a multilayer prepreg stack; placing saidmultilayer prepreg stack into a heated mold of a molding press; flowmolding the prepreg stack by at least partially closing the mold to forma SMC component; thermally curing said SMC component in said mold,wherein said mold is closed during curing; forming at least oneadditional SMC component from at least one additional multilayer prepregstack, each said at least one additional multilayer prepreg stackincluding an additional reference blank having said reference blankshape and said reference blank surface-area content; and weighing eachadditional reference blank separately after cutting; wherein the excesssurface-area content is mathematically determined in accordance with arelationship between the excess surface-area content, the referenceblank surface-area content, the reference blank weight, and thepredetermined non-reference blank weight; and wherein the non-referenceblanks are cut from a portion of the prepreg web directly adjacent tothe reference blank.
 2. The method as claimed in claim 1, wherein theblanks for said multilayer prepreg stack and said one or more additionalmultilayer prepreg stacks are cut from said single prepreg web.
 3. Themethod as claimed in claim 1, wherein the non-reference blanks in saidmultilayer prepreg stack are rectangular and congruent in relation toone another, wherein a dimension (l) of the side of the rectangle is thesame for the non-reference blanks of said multilayer prepreg stack andfor the non-reference blanks of said at least one additional multilayerprepreg stack, and wherein a dimension (b) transverse to the dimension(l) is dimensioned individually for the non-reference blanks in eachmultilayer prepreg stack in accordance with the relationshipb=F_(r)·(G_(g)/G_(r,actual)−1)l·n, where n denotes the number ofnon-reference blanks.
 4. The method as claimed in claim 3, wherein thedimension (l) is aligned parallel to the longitudinal direction of theprepreg web and the dimension (b) is aligned transversely to thelongitudinal direction of the prepreg web.
 5. The method as claimed inclaim 1, wherein the surface area of the reference blank isapproximately 20 to 60% of a total surface area of all blanks of themultilayer prepreg stack.
 6. The method as claimed in claim 1, whereinthe blank lying at the bottom of the multilayer prepreg stack is thereference blank.
 7. The method as claimed in claim 1, wherein thereference blank and the non-reference blanks of the multilayer prepregstack are cut out next to one another from the prepreg web, transverselyto the longitudinal direction of the prepreg web, in such a way that thespace required in the longitudinal direction of the prepreg web is assmall as possible.
 8. The method as claimed in claim 1, wherein saidcutting is performed by a robot-guided cutting tool, and wherein theprepreg web rests on a hard base.
 9. The method as claimed in claim 1,wherein said cutting is performed by a circular saw blade, said circularsaw blade performing a rotary oscillating movement with more than 15,000cycles per minute, and also having rotary oscillating displacements,wherein said displacements are greater than a tooth pitch of saidcircular saw blade.
 10. The method as claimed in claim 9, wherein saidcircular saw blade performs a rotary oscillating movement having fromabout 20,000 to about 30,000 cycles per minute.
 11. The method asclaimed in claim 9, wherein said displacements are smaller than thethickness of the prepreg web.
 12. The method as claimed in claim 9,wherein during the cutting by the rotary oscillating circular saw blade,the prepreg web is supported by a smooth, continuous base that is freefrom joints, wherein said base is composed of a harder material than thecutting teeth of the circular saw blade.
 13. The method as claimed inclaim 1, wherein the prepreg web is stripped of coverings or protectivefilms prior to cutting.
 14. The method as claimed in claim 1, whereinthe relationship is:F _(a) =F _(r)·(1−G _(n) /G _(r, actual)); wherein F_(a) is the excesssurface-area content of the single prepreg web; F_(r) is the referenceblank surface area content; G_(n) is the predetermined non-referenceblank weight; and G_(r,actual) is the reference blank weight.
 15. Themethod as claimed in claim 1, wherein the relationship is:F _(u) =F _(r)·(G _(g) /G _(r,actual)−1); wherein F_(u) is a surfacearea content to be maintained by the non-reference blanks correspondingto the reference blank; F_(r) is the reference blank surface areacontent; G_(g) is the predetermined non-reference blank weight in total;and G_(r,actual) is the reference blank weight.